Decimation glitch for ratio < 0.2
The ratio on the decimate modifier doesn't refer to level detail, it refers to the number of faces. From the wiki:


  Ratio: The ratio of faces to keep after decimation, from 0.0 (0%, all faces have been completely removed) to 1.0 (100%, mesh is completely
  intact, except quads have been triangulated). 
      As the percentage drops from 1.0 to 0.0, the mesh becomes more and more decimated until it no longer visually looks like the original
  mesh.